Recent automatic transmissions or continuously variable transmissions have been demanded to be light and small and sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. There are some automatic transmissions or continuously variable transmissions that control the lock-up clutch built in the torque converter to be slipped at a low velocity (slip lock-up control). These transmissions have been improved with the slip lock-up control so that the drive feeling can be improved by absorbing a torque variation and the engine torque can be transmitted to the transmission mechanism efficiently. Some of the continuously variable transmissions are provided with a wet starting clutch which is initially allowed to slip and then coupled so as to start the vehicle smoothly from the halt, that is a so-called slip control. Lubricating oils used for the transmissions wherein slip control for the lock-up clutch or starting clutch is carried out are required to provide excellent torque capacity and less shifting shock as well as to have excellent and long-lasting initial anti-shudder properties.
Continuously variable transmissions include a belt type CVT comprising a driving pulley, a driven pulley, and a belt for transmitting a power which belt is composed of a plurality of elements (hereinafter referred to as “tops”) and a belt (steel belt) or chain connecting the elements together. Lubricating oils for such a belt type CVT have been required to have cooling, lubricating, and anti-wear properties and be enhanced in friction coefficient between the metal pulleys and the belt to obtain excellent power transmission capability.
In automobiles equipped with a belt type CVT, a phenomenon that belt noise generates may occur. It has been found that this phenomenon is caused by gear meshing sounds of the gears arranged behind the CVTs, resulting from a fluctuation in the rotation of the driven pulley. It has also been found that such a fluctuation occurs when the change in the friction coefficient (p) of the belt and tops over the change in the slipping velocity (V) is in a negative gradient (for example, see Patent Literature 1).
Another noise generates when the elements meshes into the pulleys.
Therefore, it is important for a belt type CVT and a continuously variable transmission having a slip-controlled wet clutch such as a lock-up clutch or a starting clutch to maintain anti-shudder properties for a long period of time and exhibit the metal-to-metal μ-V characteristics of positive gradient to prevent the generation of belt noise, while still maintaining the metal-to-metal friction coefficient between the metal pulleys and the metal belt high (maintaining the power transmission capability high).
Conventionally, as an transmission oil composition that can be improved in anti-belt-noise properties, a metal-to-metal friction coefficient and the durability of anti-shudder properties, Patent Literature 1, for examples, discloses an oil composition for continuously variable transmissions comprising a base oil blended with a sulfonate, an ashless dispersant, an acid amide, an organomolybdenum compound and an amine-based anti-oxidant such that the transmitted power can be kept large, the scratch phenomenon can be prevented, and furthermore the μ-V characteristics of positive gradient can be kept for a long time. Patent Literature 2 discloses a lubricating oil composition for automatic transmissions containing phosphorus:calcium:boron:sulfur in a specific elementary ratio so that the scratch phenomenon can be prevented for a long time. Patent Literature 3 discloses a continuously variable transmission lubricating oil composition containing an organic acid metal salt with a specific composition, an anti-wear agent, and a boron-containing succinimide, as essential components, to have both a higher metal-to metal friction coefficient and anti-shudder properties for a slip control mechanism. Patent Literature 4 discloses a long-lasting continuously variable transmission lubricating oil composition comprising calcium salicylate, a phosphorous-containing anti-wear agent, a friction modifier, and a dispersant type viscosity index improver, which composition has both a higher friction coefficient between metals and anti-shudder properties for a slip control mechanism. Patent Literature 5 discloses an automatic transmission fluid composition comprising calcium sulfonate, phosphorous acid esters and further a sarcosine derivative or a reaction product of a carboxylic acid and amine, which composition has long-lasting anti-shudder properties for a slip lock-up mechanism and long-lasting properties to prevent scratch noise in a belt type CVT.
However, the anti-belt-noise properties or anti-shudder properties and the metal-to-metal friction coefficient are in a trade-off relationship, and thus there is room for further improvements from the viewpoint of these properties.